This invention relates to methods and apparatus for indicating the occurrence of an abnormal system parameter and, more particularly, relates to improved methods and apparatus for indicating the presence of an object in a body of water such as that contained in a swimming pool or the like.
It is well known that many private residences now have outdoor swimming pools located on the premises. In addition, many garden-type apartment houses and other multi-dwelling rental properties now have such pools available for use by their tenants. It is further well known that swimming pools constitute a hazard, not only for non-swimmers who may accidentally venture beyond wading depths, but also for experienced and normally capable swimmers who may become incapacitated by reason of accident or other sudden disability.
Because of these facts, it is conventional for the owners or operators of publically attended swimming pools to ban access thereto except when a "lifeguard" or other responsible attendant is available. Similarly, it is conventional for the owners of residential pools and the like to maintain such a restriction. On the other hand, it is obvious that the average owner of a residential or apartment house property does not have the capability of maintaining a 24-hour surveillance of his pool.
Because of this, and because of the fact that any swimming pool will potentially attract unauthorized users, many devices have been proposed for the purpose of generating an alarm in the event the pool is entered either accidentally or intentionally but without authorization. Security systems of the type intended to guard commercial establishments and the like are generally unsuitable for protecting a swimming pool, not only because they are far too complex and expensive, but also because such systems themselves require attendance by specially trained personnel. On the other hand, it will readily be apparent that any swimming pool alarm which is capable of attaining its purposes must itself be secure from being disconnected or disabled by unauthorized persons. In addition, however, such a system must not be subject to false alarms such as may be caused by wind blowing on the surface of the water in the pool.